In this specification the term “light” will be used in the sense that it is used in optical systems to mean not just visible light but also electromagnetic radiation having a frequency between 100 THz and 375 THz.
In an optical fibre communication system, which uses light of a single frequency, the specific frequency of the laser source is not critical provided that it falls within the low insertion/dispersion characteristics of the optical fibre and bandwidth of the receiver. Provided that the bandwidth of the receiver is broad enough it will be able to detect a modulated signal if the laser source should drift or vary for any reason.
The frequency drift or variation may be due to operating temperatures, physical construction of the laser, and ageing characteristics of the laser materials. A distributed feedback (DFB) laser will shift about 10 GHz per ° C., in the C-Band communications band. Laser sources are therefore typically provided with temperature control devices.
The information carrying capacity of optical fibre communication systems can be increased by the use of wavelength division multiplex (WDM) systems in which a number of different frequency channels are carried over a single fibre. In WDM systems drift of the frequency channels constrains the number and spacing of the different channels and hence the data carrying capacity of the system.
There are two principal communication bands nominally centred on 231 THz (1300 nm) and 194 THz (1550 nm). The 194 THz band is the more utilised band because of its suitability for a variety of different generic communications applications. The 194 THz (1550 nm) WDM systems are presently evolving into systems comprising eighty (80), 2.5 Gb/s channels, and into forty (40), 10 Gb/s channels. The 194 THz communications band is located in the IR spectrum with International Telecommunication Union (ITU) channels spacings (ITU Grid) of 200, 100 and 50 GHz spread between 191 THz and 197 THz. The operating life ITU channel frequency stability specification for 194 THz (1550 nm) communication systems is typically set at 1.25 GHz variation over the operating life.
To provide 40 or 80 channels within the 194 THz band requires the use of a light source that can be accurately set to specific frequencies and be maintained at those frequencies (within limits) over the operating life. Suitable sources include distributed Bragg reflector (DBR) lasers each of which is operated to produce light of one frequency channel with means for selecting a required channel. Wide range tunable lasers have also been developed such as sample grating distributed Bragg reflector (SG-DBR) lasers as is described in Chapter 7, “Tunable Laser Diodes”, Markus-Christian Amann and Jens Buus, Artech House, ISBN 0-89006-963-8. The tuning mechanism of SG-DBR lasers is by means of differential current steering of the operating frequency by means of currents supplied to the front and rear sample gratings of the Bragg reflector sections, with fine tuning being possible by means of the supply of a control current to the phase section. In general semiconductor lasers can be dynamically tuned either by means of current drive(s), electric field(s) control, or temperature control.
In order to prevent drift of a semiconductor laser's frequency devices are commercially available that perform the function of “frequency locking”. One known device used for frequency locking is a Fabry Perot etalon filter, which is described in a paper entitled “A Compact Wavelength Stabilization Scheme for Telecommunication Transmitters”, by B. Villeneuve, H. B. Kim, M. Cyr and D. Gariepy, published by Nortel Technology Ottawa, Canada.
A method of frequency locking is disclosed in U.S. Pat. No. 5,789,859 which describes a method in which an input signal is passed through a Fabry Perot etalon to provide a detected output signal having an intensity that varies with wavelength. A reference signal taken separately from the input signal is compared with the output signal detected from the etalon to provide a feedback signal that corresponds to the frequency of the input signal. The system is then calibrated to determine a ratio of intensities that determines a locked state. The frequency of the input signal can then be adjusted if the ratio falls outside of predetermined ratio limits.
The present invention provides an improved arrangement for a frequency locking device using a passive frequency discriminator (PFD).